1. Field of the invention
The present invention is generally concerned with filters used to clean a fluid circulating in a network under pressure and likely to entrain debris, detritus or foreign bodies
It is more particularly directed to treatment of industrial process water which may contain man-made debris or detritus or vegetables, such as algae, for example, or animals such as mussels, shellfish or fish liable to proliferate on the downstream side unless previously separated out.
2. Description of the prior art
Filters used to clean industrial process water usually comprise a filter body and, at least in part installed in the filter body, a screen, that is to say a filter element that may comprise a screen of perforated sheet metal or filter fabric, or a screen formed by an array of parallel blades or filaments carried by transverse support bars, the screen being designed to be inserted into the flow to be treated.
The present invention is more particularly directed to the case where, to obtain self-cleaning filters, means are provided for systematically evacuating debris, detritus and foreign bodies held back by the screen.
In some implementations a mechanical cleaning device is used for this purpose.
For example, where the screen simply comprises a filter element curved into a part-cylindrical shape there is disposed in its vicinity, and in practise in a counterflow arrangement, that is to say on the upstream side relative to the corresponding flow, a suction device which is connected to the drain system and which is mounted to rotate about the axis of the screen.
Self-cleaning filters equipped in this way with a rotary cleaning device have numerous disadvantages.
First of all, they usually cause significant turbulence on the downstream side and therefore constitute a source of head losses there.
They are also relatively complex and, inevitably having regions where the velocity is very high, they are relatively vulnerable to erosion and corrosion.
Finally, and most importantly, as the rotary cleaning device fitted to them is disposed on the upstream side of the screen and therefore directly in the flow to be treated, they are subject to frequent malfunctions, the rotary cleaning device being jammed by debris, detritus or foreign bodies conveyed by the flow or already intercepted by the screen.
In currently known self-cleaning filters of this type any such jamming may damage or even break the cleaning device and/or the screen.
In other implementations, which have the advantage of using no moving parts for the most part, except for internal valves and/or flaps necessary for their operation, the direction of flow is systematically reverse at appropriate times in order to achieve the necessary elimination of debris, detritus and foreign bodies held back by the screen.
However, this implies the temporary removal from service of either the filter as a whole or at least one compartment of the filter, which is detrimental to the overall efficiency.
In other implementations which have the advantage of comprising no moving parts at all the required elimination of debris, detritus and foreign bodies held back by the screen is achieved very simply by structural design features which cause the incoming flow to sweep across the screen.
For example, in a first type of implementation of these self-cleaning filters without moving parts the arrangement is such that the speed vector of the incoming flow is oblique to the surface of the screen and thus has not only a component perpendicular to the latter, necessary to have the flow pass through the screen, but also a tangential component; by virtue of this latter component any debris, detritus and foreign bodies held back by the screen are systematically and continuously entrained.
In this first type of implementation of self-cleaning filters using no moving parts the filter body is in practise disposed around the screen in the manner of a spiral staircase, the flow to be treated entering the filter body tangentially and the treated flow leaving it axially.
Although giving satisfaction, the corresponding implementations are for the most part somewhat bulky and since they change the orientation of the flow to be treated, which also and inevitably results in head losses on the downstream side, they can be difficult to install, especially in existing installations, and so are not suitable for all applications.
In a second type of implementation of self-cleaning filters using no moving parts the filter body is adapted to be inserted in the flow direction in the conduit carrying the fluid to be cleaned, the filter screen then having a conical filter wall which has its larger diameter end at least partially fitted within the filter body, there being in the filter body and around the screen a volute-form collection space to which is connected an offtake conduit for evacuating debris, detritus and other foreign bodies held back by the screen.
However, in this second type of self-cleaning filter with no moving parts the screen is in practise cleaned only during periodic purges, the feed water for such purging then causing the incoming flow to rotate to bring about the necessary cleaning action; these periodic purges, which are diverted to the drainage system, inevitably mobilize large flowrates of water which is detrimental to a regular feed to the downstream installation.
Also, in the corresponding implementations it is currently necessary, to ensure an acceptable ratio between the filter surface area of the screen and the cross-section of the conduit into which it is inserted, for the length or height of the screen along the axis of the conduit to be considerable, in practise very much greater than twice the diameter of the conduit; once again this makes the possiblity of installing such filters very random, especially in existing installations, where there are rarely sufficient lengths of conduit between two bends available.
A general object of the present invention is an arrangement which, for a self-cleaning filter without moving parts adapted to be inserted in the flow direction into a conduit, has advantages including the fact that it advantageously makes it possible to minimize the overall length of the filter along the axis of the conduit whilst providing an acceptable ratio between the filter surface area and the cross-section of the conduit, and whilst also procuring at all points on the screen an angle of incidence relative to its surface of the speed vector of the flow to be treated adapted to procure systematic entrainment towards a collection space of debris, detritus or foreign bodies held back by the screen.